This invention relates to optical apparatus which operates selectively on optical signals based on the frequency content of the signal. Illustratively, the invention relates to optical filters, channel balancers and dispersion compensators.
One architecture for optical transmission systems, known as wavelength division multiplexing (WDM), involves assigning a different wavelength to each of a plurality of signal channels. At certain terminals or stations of a WDM system it is necessary to select a particular channel and separate it from the others. This function is performed by what is commonly known as a channel dropping filter.
D. C. Johnson et al, Electronic Letters, Vol. 23, p. 668(1987) have proposed such a filter or tap based upon the use of an optical grating in each of a pair of parallel optical fiber (or integrated-optic) arms disposed between a pair of 3 dB couplers. The fiber arms are supposed to be of equal length and the gratings are supposed to be in registration with one another so as to introduce no phase shift between different modes. In contrast, the 3 dB couplers themselves introduce a 90.degree. phase shift. Consequently, as shown in FIG. 1 of the article, reflected light .lambda..sub.o traverses paths 1 and 2, undergoes constructive interference and exits port B. However, any light attempting to exit port A would experience destructive interference. This design is difficult to manufacture because the performance of this type of coupler is critically dependent on the length of the coupling region. If the length is not ideal, then the filter may exhibit undesirably high levels of backreflected power and low levels of transmitted power. This problem is exacerbated if the filters are cascaded because the losses are cumulative.
R. C. Alferness et al, Electronic Letters, Vol. 24, No. 3, pp. 150-151 (1988) describe an integrated optic version of the Johnson filter implemented in InP/InGaAsP. They point out that careful alignment of the gratings is critical to insure equal path lengths over the two grating arms. Otherwise, some of the light at .lambda..sub.o will be reflected back into the incident waveguide (waveguide A of Johnson et at; upper left waveguide of Alferness et al).
Another problem associated with many optical transmission systems is wavelength dispersion; that is, systems which utilize a dispersive transmission medium, such as a standard 1.5 .mu.m optical fiber, cause optical radiation of different wavelengths to travel at different velocities. Consequently, a digital poise tends to spread out as it propagates, thereby limiting the bit rate of the digital system. While numerous techniques have been proposed for addressing dispersion, including the use of single frequency lasers and/or dispersion shifted fiber, the problem remains.